If I am playing metal (genres: modern metal, prog metal, post metal) I slap all of the time. Probably every song on my last album aside from one where the bass line was all tapping. If you are a fingerstyle metallist it's a phenomenal way to get a definitive attack, punch, and grindy sound on low notes. It enhances the overall aggression of every band that I play in. IMO the fluffy subwoofer sound of light fingerstyle on the B string doesnt add much, just low end. When I slap it cuts through both the kick and guitars and people realize there is a bassist involved.
I never use it unless things call for it. I have also spent years playing in bands (hip hop, lighter rock) where I never had to slap anything and used a light touch in general.